The present invention relates generally to a photocatalyst and relates particularly, though not exclusively, to a photocatalyst suitable for use in a slurry-type photoreactor for degradation of organic compounds or chemicals such as but not restricted to those occurring in waste water effluents.
Organic chemicals which may be found as pollutants in waste water effluents from industrial or domestic sources, must be removed or destroyed before discharge to the environment. Such pollutants may also be found in ground and surface waters, which also require treatment to achieve acceptable drinking water quality. Much of the natural purification of aqueous system lagoons, ponds, streams, rivers and lakes is caused by sunlight initiating the breakdown of organic molecules into simpler molecules and ultimately to carbon dioxide and other mineral products. There are various natural sensitisers that accelerate this natural purification process. The utilization of xe2x80x9ccolloidal semiconductorsxe2x80x9d and the introduction of catalysts to promote specific redox processes on semiconductor surfaces has been under development since 1976. Laboratory studies have since confirmed that naturally occurring semiconductors enhance this solar driven purification process.
The photocatalytic detoxification or degradation of wastewater is a process that combines heterogeneous catalysis with solar technologies. Semiconductor photocatalysis, with a primary focus on titanium dioxide, has been applied to a variety of problems of environmental interest in addition to water and air purification. The application of illuminated semiconductors for degrading undesirable organics dissolved in air or water is well documented and has been successful for a wide variety of organic compounds. Organic compounds such as alcohols, carboxylic acids, amines, herbicides and aldehydes, have been photocatalytically destroyed in laboratory and field studies. The solar photocatalytic process can also be applied to destroy nuisance odours, taste and odour compounds, and naturally occurring organic matter, which contains the precursors to trihalomethanes formed during the chlorine disinfection step in drinking water treatment. It is thus generally recognised that the photocatalytic process can mineralise hazardous organic chemicals to carbon dioxide, water and simple mineral acids.
Many processes have been proposed over the years and are currently used to destroy organic toxins. Current treatment methods for destruction or degradation of these organic toxins, including adsorption by activated carbon and air stripping, merely concentrate the organics present by transferring them to the adsorbent or air. However, these known treatment methods do not convert the organic toxins into non-toxic wastes. Thus, one of the major advantages of the photocatalytic process over these existing treatment methods is that there is no requirement for further treatment or disposal methods. Another advantage of the photocatalytic process is that when compared to advanced oxidation technologies, especially those using oxidants such as hydrogen peroxide and ozone, expensive oxidising chemicals are not required as ambient oxygen is the oxidant in the photocatalytic process. Photocatalysts are also self regenerated and can be reused or recycled.
It is understood that during the photocatalytic process the illumination of a semiconductor photocatalyst with near ultraviolet radiation activates the photocatalyst establishing a redox environment in an aqueous solution. Semiconductors act as sensitisers for light induced redox processes due to their electronic structure which is characterised by a filled valence band and an empty conduction band. The energy difference between the valence and conduction band is called the bandgap. Several semiconductors have band-gap energies sufficient for catalysing a wide range of chemical reactions. Titanium dioxide is the semiconductor most thoroughly investigated in the literature and is believed to be the most promising for photocatalytic destruction of organic chemicals. It is generally acknowledged that titanium dioxide provides the best compromise between catalytic performance and stability in an aqueous media.
In the photocatalytic detoxification or degradation of, for example, waste water, a large area of reactive photocatalyst surface is required for the large volume of water to be treated. Accordingly there is a need for submicron photocatalyst particles. Generally the photocatalytic reaction is conducted in a suspension of the insoluble semiconductor photocatalyst and, therefore, an additional separation step is required to remove the catalyst from the treated water. The removal of such fine photocatalyst particles from large volumes of water is difficult and involves additional expense in a continuous treatment process. In one reported study charge neutralisation, coagulation and flocculation was investigated as a basis for the enhanced settling and recycling of titanium dioxide in a suspended-photocatalyst system. Although relatively effective, such an approach includes the additional cost for chemicals and a requirement for critical process control. In addressing these problems, research has been carried out by immobilising titania onto various bearing materials, such as glass beads, sand, silica gel, quartz optical fibres, glass fibre in the form of mesh, or on a glass reactor wall. In this research the semiconductor photocatalyst is fixed to a stationary illuminated support and waste water passed over or through the immobilised catalyst bed. This approach has the distinct disadvantage of greatly reducing the available surface area of the catalyst as well as slowing down the photocatalytic reaction due to mass transfer limitations.
An intention of the present invention is to provide a photocatalyst that is relatively effective in degrading organic compounds whilst being relatively stable.
According to one aspect of the present invention there is provided a photocatalyst comprising:
a core being at least partly formed of a magnetic material;
an insulative layer disposed about the core, the insulative layer being formed of an electrically non-conductive material; and
an outer layer disposed about the insulative layer, the outer layer being formed of a semi-conductor having photocatalytic properties.
According to another aspect of the present invention there is provided a method of forming a photocatalyst comprising the steps of:
forming an insulative layer about a core which is at least partly formed of a magnetic material, the insulative layer being formed of an electrically non-conductive material; and
forming an outer layer about the insulative layer, the outer layer being formed of a semi-conductor having photocatalytic properties.
Preferably the step of forming an insulative layer involves exposing the core to a reaction medium and adding appropriate reagents to precipitate the insulative layer on the core. In one embodiment the step of forming a silica-based insulative layer involves suspending the core in a reaction medium of ethanol and adding aqueous tetraethoxysilane in the presence of ammonium hydroxide to precipitate silica on the core.
Alternatively the insulative layer and/or the outer layer may be formed by applying the Chemical Vapour Deposition (CVD) technique or by using Flame Aerosol Technology.
Typically the method of forming the photocatalyst further comprises the step of photodepositing a metal on the outer layer so as to enhance its photocatalytic properties. More typically the step of photodepositing the metal involves mixing the photocatalyst with a solution of metal ions and exposing the resultant suspension to natural or artificial radiation so as to reduce the metal ions and deposit them as the solid metal on the outer layer. In one embodiment the photocatalyst is mixed with a solution of silver nitrate.
According to a further aspect of the present invention there is provided a method of degrading organic compounds in an aqueous solution comprising the steps of:
providing a suspension of a photocatalyst in the aqueous solution, the photocatalyst comprising a core being at least partly formed of a magnetic material, an insulative layer disposed about the core, the insulative layer being formed of an electrically non-conductive material, and an outer layer disposed about the insulative layer, the outer layer being formed of a semi-conductor having photocatalytic properties;
exposing the aqueous solution together with the photocatalyst to natural or artificial radiation so as to promote the degradation of the organic compounds; and
promoting separation of the photocatalyst from the aqueous solution via magnetic separation of the catalyst.
Generally, the insulative layer is formed predominantly of silicon dioxide or silica or a derivative thereof. It is understood that the insulative layer prevents the contact of the magnetic core with the photocatalytic outer layer which ordinarily results in a photo-induced dissolution of the core.
Preferably the magnetic core is formed predominantly of iron oxide or a derivate thereof such as magnetite or maghemite.
Typically the photocatalytic outer layer is formed of titanium dioxide or titania (in the form of anatase crystalline phase) or a derivative thereof.
Preferably the photocatalyst further comprises a metal photodeposited on the outer layer so as to enhance its photocatalytic properties. More preferably the metal is silver. Additionally the metal includes but is not limited to copper or gold.
Generally the natural or artificial radiation is near ultra violet radiation having a wavelength less than 400 nm.
Typically the outer layer is at least partly formed of titanium dioxide or titania in an anatase crystalline form or a derivative thereof.